This invention relates to the purifying of crystal material, the doping of the material and the growth of crystals.
Bridgeman, Bridgeman-Stockbarger, Czochralski and variations have been used for crystal growth. Depending on the crystal growth method, the crystal type and the crystal size, one has to overcome sets of problems. This invention relates to the purification of the crystal material and the crystal growth process itself.
Crystal size and the quality of the crystal starting material play important roles in the production of scintillation crystals. The starting material labeled "scintillation grade" is of five 9's purity 99.999%. Often the starting material has poor stoichiometry ratio. Growing crystals in a closed type system that have large diameters and up to over 2000 pounds in weight result in crystals that have poor crystal quality. Crystal purity, dopant distribution, defect density and distribution and built-in stress imposed on the crystal during the crystal growth process and the crucible removal may be at unacceptable levels. With the exception of small crystal portions grown at the beginning of the crystal growth, crystals may have lower purity than the starting material. Dopant concentration varies dramatically. That in turn creates uneven light output and decreases the energy resolution of scintillation crystals. When handling large size crystals during the hot transfer, the crystals release large portions of iodine and thallium iodine vapors. Exposure to ambient temperature creates various defects and defect densities in the hot crystals.
The current practices where large barrel-shaped crystals are grown for all applications, regardless of the fact that most applications use rectangular shapes, makes the yields rather low. Scaling up crystal plate sizes from 0.5-1 inch thick slabs cut perpendicular to the crystal length of a barrel-shaped crystal requires large financial investments. At the same time increasing slab geometry increases the crystal production cost by decreasing the growth rate and lowers the crystal quality and yield.
Existing purification methods include supplying a gaseous medium to a surface of a melt carried in a crucible. Those methods require extended times for purification, up in the range of 96 hours. Those methods also ineffectively cure the melt, as lower portions of the melt are never purified.
During melt purification, impurities react with the gas molecules and exit the melt in a gaseous phase. Some impurities react and precipitate from the melt as a sludge. Other reacted impurities float to the surface.
Needs exist for purification systems that remove impurities faster and more efficiently.
These problems and many more remain in the present practices. Needs exist for new approaches for crystal material purification and the crystal growth processes.
Purifying of crystals by reactant gas contact in current systems results in delays and adds significant times to the crystal growth process.